The Diencephalon

The diencephalon extends from the region of the mamillary bodies and the posterior commissure at its caudal end to the interventricular foramen at its most rostral end. It forms the lateral wall of the third ventricle and is made up principally of the hypothalamus, epithalamus, thalamus, and subthalamus (also termed ventral thalamus). The thalamus lies above the hypothalamic sulcus, and the hypothalamus below it. The thalamus makes up the dorsal wall and the hypothalamus the ventral wall of the ventricle. Little can be seen of the diencephalon, since most of it is surrounded by the cerebral hemispheres, and it is best seen in sagittal section. The only part that is visible on the brain surface is in the ventral view, when the infundibulum, bilateral mamillary bodies and the tuber cinereum can be seen, as well as a surface rostral boundary, the optic chiasm. The mamillary body holds the mamillary nuclei of the hypothalamus.

In sagittal section, the hypothalamus is seen from the mamillary body at its caudal end to the interventricular foramen ros-trally. Functionally, the hypothalamus is critical for normal life, since it controls body temperature, fluid and water balance, and neuroendocrine function, and has an important role in the control of the autonomic nervous system and emotional and sexual behavior. At the base of the hypothalamus is the infundibulum or pituitary stalk, which connects the hypothalamus to the pituitary gland through blood portal and nervous links (see also p. 291). Several small but important nuclei have been identified in the hypothalamus.

The thalamus is the largest member of the diencephalon, and if it were dissected free might resemble a hen's egg in shape (see also p. 29). It is separated from the hypothalamus by a groove, the hypothalamic sulcus. There are two thalami, joined by a massa intermedia or interthalamic adhesion. The thalamus is a huge relay station, and has massive reciprocal connections with the cerebral cortex. The thalamus extends forward to the interventricular foramen, and is bounded laterally by the posterior limb of the internal capsule (see p. 35), and the head of the caudate nucleus. Internally, the thalamus consists of several nuclei, which project to the ipsilateral cerebral cortex, and the cortex in turn sends reciprocal fibers back to the areas from which it received them. Functionally, this relationship serves to control the organism's response to inputs from the special and the general senses, and to ensure a proper motor response to them.

Immediately below the thalamus lies the subthalamus, which is situated dor-solaterally to the hypothalamus. The epithalamus consists of the habenular nucleus and the pineal gland (see p. 9). The pineal gland synthesizes the hormone me-latonin, which may modulate sleep-waking rhythms, and in recent years mela-tonin has been advocated to alleviate the condition known as jet lag.

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